During integrated circuit processing, multiple layers are placed above each other on a wafer. The layers have electrically conducting connections called via's. In order to make sure that the layers are connected by the via's, alignment marks are used to position the layers. In a first layer a first alignment mark is created together with a first circuit pattern. The wafer is subjected to several process steps which include depositing a second layer on the first layer. Then, the exact position of the first circuit pattern in the first layer is determined by measuring the position of the first alignment mark. A second circuit pattern is created in the second layer which is positioned substantially precisely on top of the first layer using the measured position of the first alignment mark. Together with the second circuit pattern, a second alignment mark is formed which will be used when a third circuit pattern will be created in a third layer deposited in a later process step on the second layer.
Forming a circuit pattern or an alignment mark in a layer is done in a lithographic process. In the lithographic process, a photoresist layer is applied over the wafer and the wafer is positioned in an image plane of a projection lens. A reticle is brought into an object plane conjugate to the image plane. The reticle is used to pattern an incident beam of radiation to comprise the circuit pattern or an alignment mark pattern in its cross section. With the wafer in the image plane, the photoresist layer is exposed to an image of the circuit pattern or the alignment mark pattern.
The wafer is then subjected to several processing steps, such as baking, etching, and applying a new photoresist layer.
Chemical-mechanical polishing (CMP), is a form of polishing that is frequently used in the several processing steps of integrated circuit processing. During CMP, a slurry is rubbed over a top layer of the wafer so that the thickness of the top layer is reduced. The polishing effect is intensified by a chemical reaction which weakens the top layer. However, chemical-mechanical polishing (CMP) has a number of drawbacks, for example, in the common application during the so called shallow trench isolation process. In this process, a substrate, having a first surface, is provided. A series of trenches in the substrate (including the trenches of an alignment mark) are blanketed under an oxide layer that fills the trenches. The oxide layer is polished back to the first surface using CMP. Because the substrate is treated by CMP after the trenches are filled, the surface after CMP is flat, in other words, the topology of the alignment mark is lost. If the alignment mark is then overlaid with a non-transparent layer, the alignment mark is lost: the topology is lost and the alignment mark is not visible anymore either.
DE 102 59 322 B4 explains (FIG. 1) that this problem can be overcome by making the trenches of the alignment mark wider and deeper so that they are not fully filled by the oxide layer. The trench (10) of the alignment mark is filled by an oxide layer (50). When the structure is subjected to CMP, the result is a small dip in each alignment trench (FIG. 2). However this causes erosion (101) which affects neighboring circuits. This erosion is in turn prevented by applying a sacrificial layer over the oxide layer (FIG. 3). The sacrificial layer (30) fills the dips and is thick enough to be higher than the level (91) of the trench edges. When polishing back the oxide layer at a later process step, the CMP process is applied to a flat surface all the time and will not lead to erosion, which is shown in FIG. 4. The sacrificial material (30) in the dip is then removed so that there is topology of the alignment mark in the resulting top surface. The sacrificial material is removed by etching, and care is desirable so that the oxide layer is kept in place, i.e. is not removed. Finally (FIG. 5), after overlaying the first surface again with a non-transparent layer (100), the trench of the alignment mark corresponds to a dip (201) in the non-transparent layer. This dip (201) in the non-transparent layer can be used for alignment.
A drawback of this method is that it requires widening of the trenches of the alignment mark. It is desirable to have the trenches of the alignment mark as wide as the trenches of circuit patterns. This is desirable because of inaccuracies when bringing the alignment mark and the circuit patterns from a reticle onto a wafer by imaging. As explained earlier, for forming an image of the alignment mark and the circuit patterns, a projection lens is used. The combination of aberrations of the projection lens and a difference in width between the trenches of the alignment mark and the trenches of the circuit patterns will lead to an error in distance between the alignment mark and the circuit patterns in the image. This error in distance leads to alignment errors. In order to produce the circuit patterns at the lowest possible cost, the circuit pattern trenches are made as small as possible. Making the trenches of the alignment mark as wide as the circuit pattern trenches therefore does not allow for wide alignment mark trenches as described in DE 102 59 322 B4.
Another drawback of this method is that it requires the trenches of the alignment mark to be deeper than the trenches of the circuit pattern. To create trenches in one layer with two different depths, at least two different etching steps are required. Applying two different etching steps is more expensive and requires more time than applying one etching step.
Application of CMP in the so called W-CMP flow (wherein W stands for Tungsten and wherein CMP stands for chemical mechanical polishing) is schematically shown in FIGS. 6a-6d. For producing an alignment mark in combination with W-CMP flow, a 1st pattern (not shown) and a 2nd pattern of recesses (1030), (see FIG. 6a), is etched into an oxide layer (1010) which covers a wafer (1000). The 1st pattern of recesses corresponds to a circuit pattern and the 2nd pattern of recesses (1030) corresponds to an alignment mark. Then, the oxide layer (1010) is covered (FIG. 6b) by a Tungsten layer (1040), thereby filling the 1st and 2nd patterns of recesses (1030) in the oxide layer (1010). The Tungsten in the 1st pattern of recesses (1030) will function as conducting material of a “via” later in the finished product to be made out of the wafer (1000) under process.
Then, the wafer (1000), with covering layers, is subjected to a chemical-mechanical polishing (CMP) step (FIG. 6c) to remove all Tungsten that is not located in the recesses (1030). Since the oxide layer functions as an electrical insulator, conduction of an electrical current will only be possible via the conducting material in the recesses (1030) and not next to the recesses. In this way, at the recesses (1030), an electrical contact can be made to a layer of the wafer (1000) below the oxide layer (1010).
At the 2nd pattern height differences between the Tungsten and oxide are created by the CMP. Although the Tungsten is not completely removed from the recesses (1030), the level of the remaining Tungsten in the recesses is lower than the level of the remaining oxide. Alignment to the 2nd pattern depends on the height differences after CMP.
However, CMP also causes erosion (101) of the oxide layer (1010) and Tungsten at the 2nd pattern of recesses. The erosion (101) leads to a damaged alignment mark (shown in the circles in FIG. 6c). A damaged alignment mark leads to alignment errors, which will be explained below.
Following the CMP, an Aluminum layer (1070) is deposited (shown in FIG. 6d). The Aluminum layer is deposited so that a first surface is disposed on a contact surface of the oxide layer (1010) and a free surface is disposed opposite to the first surface. When depositing the Aluminum layer (1070), the height differences between oxide and Tungsten at the alignment mark are transferred to height differences at the free surface, thus forming the alignment mark in the free surface of layer 1070. Aligning to the Aluminum layer (1070) depends on height differences of the free surface because Aluminum is opaque to alignment radiation.
Damage to the alignment mark in the oxide layer (1010) is also transferred to the free surface and may lead to errors in the aligned position. On the other hand, small height differences in the free surface causes low signal to noise ratios. Low signal to noise ratios result in alignment errors. At too low signal to noise ratios, alignment is not possible at all.
The effect of erosion (damage to the alignment mark) is minimized by splitting up lines in an alignment mark into segments. However, by segmenting an alignment mark, the height differences between Tungsten and oxide will be smaller after CMP than without segmenting. This is related to CMP process characteristics.
Since the height differences between Tungsten and oxide underlying the contact surface are small, the height differences of the free surface are small, which leads to low signal to noise ratios while aligning. Therefore, when introducing segmenting to overcome alignment errors related to damaged alignment markers, alignment errors related to low signal to noise ratio are introduced and, in the worst case, alignment is not even possible anymore.
On top of that, the contact surface of the oxide layer (1010) is scratched (FIG. 6c) by the CMP process both at the circuit pattern and at the alignment mark. The scratches (102) lead to lower performance of circuit patterns. To address the scratches (102), a so called oxide buff method is applied. According to the oxide buff method, the free surface of the oxide is subjected to an oxide CMP step. Where the scratches (102) have a first depth, a small part of the oxide layer (1010) corresponding to this first depth of the scratches (102) is removed. Since the properties of the oxide are optimized for performance at the circuit patterns and not optimized for being partially removed, the oxide buff method is a relatively time consuming and expensive method to take care of the scratches (102).
The present invention relates to a method comprising providing a substrate having a first side having a first layer, creating one or more recesses, depositing a filler material in the one or more recesses, thereby also creating a second layer of filler material and removing the second layer.
The present invention also relates to the use of a hard mask material as a sacrificial material in a method comprising providing a substrate having a first side having a first layer, creating one or more recesses, depositing a filler material in the one or more recesses, thereby also creating a second layer of filler material and removing the second layer.
The present invention also relates to providing a substrate having a first side having a first layer which is arranged to form an electrical resistor, etching one or more recesses, depositing a filler material in the one or more recesses, thereby also creating a second layer of filler material and removing the second layer.
The present invention also relates to an alignment mark created by a method comprising providing a substrate having a first side having a first layer, creating one or more recesses, depositing a filler material in the one or more recesses, thereby also creating a second layer of filler material and removing the second layer.
In a first embodiment of the invention, a method comprises providing a substrate having a first side having a first layer and depositing a sacrificial layer on the first layer. Then, one or more recesses are created in the sacrificial layer, the recesses at least extending into the first layer, and a filler material is deposited in the one or more recesses, thereby also creating a second layer of filler material on the sacrificial layer. Then, the second layer is removed from the sacrificial layer; and the sacrificial layer is removed by applying a first process that avoids scratching the first layer.
Depositing the sacrificial layer on the first layer does not scratch (nor polish) the first layer. The first layer is also not scratched (nor polished) by creating the second layer nor by removing the second layer from the sacrificial layer, because the sacrificial layer protects the first layer. Therefore, by applying a first process for avoiding to scratch the first layer, the first layer is not scratched during any of the steps in the method.
Since the first layer is not scratched by the method, there is no need to partially remove the first layer in order to deliver an unscratched first layer. Thereby the buff method becomes unnecessary.
Moreover, there is no need to apply an extra thick first layer in order to avoid the first layer being too thin after being partially removed by the buff method, avoiding the expense of applying an extra thick first layer.
According to a second embodiment of the invention wherein the one or more recesses each have a first part extending in the sacrificial layer, a second process is applied to remove the second layer. According to the embodiment, the sacrificial layer, the filler material, the first process and the second process are arranged to keep the filler material in the first part of the recesses in place.
Since the first part of the recesses extends in the sacrificial layer, filler material in the first part of the recesses is surrounded by the sacrificial layer. After removing the sacrificial layer, filler material kept in place in the first part of the recesses sticks out of the revealed, unscratched first layer. Arranging for a first part of the recesses to stick out of the revealed, unscratched first layer, involves choosing the appropriate combination of the sacrificial layer, the filler material, the first process and the second process. For instance, with a fixed sacrificial layer material, the higher the amount of filler material removed by the first process, the second process or both, the higher the sacrificial layer should be to end up with filler material in the first part of the recesses.
According to a third embodiment, a method is characterized by determining an aligned position of one or more features of at least one first member of a first group comprising the substrate, the first layer and the sacrificial layer. The method is further characterized by creating the one or more recesses at predetermined distances to the aligned position of the one or more features. Finally, an opaque third layer is deposited on the revealed, unscratched first layer, the opaque third layer having a free surface facing away from the substrate, thereby creating a plurality of protrusions, wherein the plurality of protrusions has associated positions corresponding to positions associated with the first part of the recesses.
The aligned position of one or more features is determined to enable creating the one or more recesses at predetermined distances to the aligned position of the one or more features. Filler material kept in place when removing the second layer and the sacrificial layer causes the formation of protrusions in an opaque third layer deposited on the first layer. The protrusions have associated positions corresponding to positions associated with the first part of the recesses. Therefore, the associated positions of the protrusions indirectly depend from the position of the one or more features. According to a feature of the invention, this indirect dependency can be used to indirectly align to the one or more features by aligning to the plurality of protrusions, even if the one or more features are obscured by the opaque third layer.
According to a forth embodiment of the invention, the first layer forms an electrical insulator and the method is characterized by extending the one or more recesses through the first layer to reach a plurality of conducting areas of the substrate. Also, an electrically conducting filler material is used in the one or more recesses and a forth, conducting layer is deposited over the revealed, unscratched layer and the filler material.
According to this embodiment, electrical connections are created from the forth, conducting layer, through an electrically isolating first layer to the plurality of conducting areas of the substrate. The connections are formed by using conductive filler material deposited in recesses that are created through the sacrificial layer and the first layer to a plurality of conducting areas. The first layer is an insulator, so that the connections are isolated from each other. Since the filler material sticks out of the revealed, unscratched first surface there is a large contact area by which the forth, conducting layer is in contact with the electrical connections. Since contact resistance scales with the inverse of the contact area, a feature of the invention is that low resistance electrical connections are provided from the forth, conduction layer to the plurality of conducting areas of the substrate.
According to a fifth embodiment of the invention, there is provided a method characterized by providing a substrate having a first side having a first layer which is arranged to form an electrical insulator and depositing a sacrificial layer on the first layer. Then one or more recesses are etched in the sacrificial layer extending through the sacrificial layer and the first layer and an electrically conducting filler material is deposited in the one or more recesses, thereby also creating a second layer of filler material on the sacrificial layer. The second layer is removed from the sacrificial layer by chemical-mechanical polishing. Finally the sacrificial layer is removed and the first layer is revealed by etching.
Depositing the sacrificial layer on the first layer does not scratch (nor polish) the first layer. The first layer is also not scratched (nor polished) by creating the second layer nor by removing the second layer from the sacrificial layer, because the sacrificial layer protects the first layer. Therefore, by applying a first process for avoiding to scratch the first layer, the first layer is not scratched during any of the steps in the inventive method.
Since the first layer is not scratched according to the steps employed by the inventive method, there is no need to partially remove of the first layer in order to deliver an unscratched first layer. Thereby the buff method is made redundant.
Moreover, added expense is avoided since there is no need to apply an extra thick first layer in order to avoid the first layer being too thin after being partially removed by the buff method.
A sixth embodiment of the invention provides for the use of a hard mask material as a sacrificial material in a method comprising providing a substrate having a first side having a first layer and depositing a sacrificial layer on the first layer. Then, according to the method, one or more recesses are created in the sacrificial layer at least extending into the first layer and a filler material is deposited in the one or more recesses, thereby also creating a second layer of filler material on the sacrificial layer. Then, still according to the method, the second layer is removed from the sacrificial layer and the sacrificial layer is removed by applying a first process for avoiding to scratch the first layer.
According to a seventh embodiment of the invention, there is provided an alignment mark created by a method comprising providing a substrate having a first side having a first layer and depositing a sacrificial layer on the first layer. Then, one or more recesses are created in the sacrificial layer at least extending into the first layer and a filler material is deposited in the one or more recesses, thereby also creating a second layer of filler material on the sacrificial layer. Then, the second layer is removed from the sacrificial layer; and the sacrificial layer is removed by applying a first process for avoiding to scratch the first layer.